Titanium and its alloys are used in the fabrication of aircraft. These metals are used to form not only the outer skin of the aircraft, but also internal support structures because of their light weight and high strength. In order to achieve desired physical properties, the titanium alloys are first heat treated. However, heat treatment results in the formation of a dense, tightly adherent oxide on outer surfaces of the metal. This oxide ranges in thickness from about 0.001 to about 0.010 inches and must be removed before subsequent machining, forming or joining operations. Scale covered parts cannot be joined by welding. Alpha case is difficult to machine, causing excessive tool wear and breakage. Also, alpha case scale can cause cracking of the titanium that may result in catastrophic failure.
Generally, in current methods, the oxide scale is removed through treatment of the metal in a series of chemical baths. Some of these chemical baths contain concentrated alkaline solutions while others contain highly toxic and corrosive acids, including nitric acid and hydrofluoric acid. As a consequence, the baths and ancillary equipment that come into contact with these corrosive chemicals must be fabricated from expensive exotic materials that are resistant to attack.
In order to remove the surface oxide scale, the heat treated titanium alloy is immersed in each of the chemical baths for a period of time. The time of immersion is estimated to allow sufficient time for the scale to dissolve in the acids, without significant intergranular attack on the underlying titanium alloy substrate. Such estimates are, at best, inexact, sometimes resulting in overimmersion accompanied by some resultant intergranular attack, or underimmersion, and at other times resulting in the retention of a residual thin scale layer. In some instances, some surfaces of the same aircraft component may be overexposed (and hence etched) on some portions of its surface, and underexposed (and hence retain a thin scale layer) at other portions of its surface. To minimize these effects, parts are sometimes designed with excessive material in areas that will be over-etched, or the areas are covered with masking composition during part of the cycle. Also, the part may be inverted at least once during the etching cycle since upper portions of the part etch faster than lower submerged portions. Some of the hydrogen generated during acid-etching may also migrate into the alloy structure causing "hydrogen-embrittlement"--a serious problem that reduces fatigue strength significantly. To minimize this problem, treated parts may have to be baked to remove the hydrogen. In addition to all of these problems, the chemical bath treatment system generates a hazardous waste containing heavy metal ions that must be disposed of in an environmentally acceptable manner. Such disposal is becoming increasingly costly.
There exists a need for a process of removing the dense oxide scale that forms on titanium and titanium alloy components used in the aircraft industry. The method should generate no, or very little, hazardous waste for disposal. Furthermore, it should be cost effective, allowing rapid cleaning of large components. The process should be controllable to avoid significant intergranular attack of the underlying metal, while at the same time completely removing the surface scale.